Composite sandwich panels are used in a wide variety of applications where high strength and light weight are required. These panels may include a lightweight honeycomb core for greater strength because of its high strength to weight ratio and resistance to fatigue failures. These panels are particularly well suited for the aircraft industry. The honeycomb cores may be made from a wide variety of materials, as can the face sheets that sandwich the honeycomb structures.
Typically, an adhesive is used to bond the face sheets to the core. The adhesive must rigidly attach the facings or skins to the core in order for loads to be transmitted from one facing to the other. If the adhesive fails, the strength of the panel is severely compromised. The adhesive is especially critical in sandwich panels which use honeycomb as the core because of the relatively small surface area over which the edges of the honeycomb contact the face sheets.
One procedure for applying composite face sheets to honeycomb involves forming a prepreg sheet that includes at least one fibrous reinforcement layer and an uncured resin matrix. Prepreg is a common term for fabric reinforcement that has been pre-impregnated with a resin system. The resin system is typically an epoxy that already includes the proper curing agent. As a result, the fabric is ready to lay into a mold without the addition of additional resin and without the steps required of a typical hand lay-up. These pre-impregnated composite fabrics and tapes, or “prepregs,” are used in a large number of aircraft applications. A film adhesive is typically added to the prepreg honeycomb core and it is then bonded to the honeycomb by curing of both the prepreg resin and adhesive resin at an elevated temperature. The film adhesive can be applied as a separate ply layer or as an integral part of the prepreg sheet.
An alternative method of bonding the face sheets to honeycomb involves applying an adhesive to the edge of the honeycomb. The adhesive is typically applied by “dipping” the edge of the honeycomb in the adhesive. The adhesives used in this type bonding are typically referred to as “dip” resins or adhesives. The advantage of this method is that the adhesive is located only where the honeycomb contacts the face sheet, rather than being distributed over the entire face sheet. This method is generally used to bond non-adhesive face sheets, such as aluminum and other metallic face sheets, to the honeycomb.
One such composite panel is the A502 Panel intended for use in primary and secondary aerospace applications, offered by Teklam Corp. of Corona, Calif. The Teklam A502 panel includes a 2024 T3 clad aluminum facings of a thickness of 0.02″ and a one quarter inch 5O52-0015-3.4 aluminum honeycomb core. This panel has an evenly distributed low resin content with an auxiliary film adhesive for bonding the layers. The film and adhesive can be represented as follows:
Prepreg (fibers with resin) with 0.02 lbs/sq ftadhesive evenly distributed throughoutFilm Adhesive at 0.030 lbs/sq. ftHoneycomb CoreThe overall weight of the reinforcement and resin in the foregoing example is 0.050 lbs/sq ft (0.02 prepreg and 0.03 adhesive). The manufacture of this panel requires cutting and laying up of the two materials onto the core, which adds to the manufacturing costs. From the foregoing, it can be seen that a layer resin is applied across the core in an even distribution despite the fact that it is not necessary to do so. The inclusion of the film adhesive is necessary but adds additional weight to the panel. There is a need in the aerospace industry for an improved panel that has a reduced weight through the optimization of the resin and adhesive weight and distribution along the core of the panel.